1. Field of the Invention
This invention relates to a solid-state image pickup device, a method for making the same, and an image pick apparatus including the solid-state image pickup device.
2. Description of the Related Art
With respect to charge transfer of CCD (charge coupled device), where the charge transfer is carried out, for example, by four-phase drive, high and low binary pulse voltages VH an VL are applied as is particularly shown in FIG. 14. In general, VH used is at 0 V serving as a reference voltage and VL is set at a voltage, which does not permit an unnecessary charge generated at the interface between a silicon substrate and a gate oxide film to be accumulated in a depletion layer beneath a transfer electrode to cause a dark current. This voltage is set at a value lower than a voltage (i.e. pinning voltage, for example, of −9.0 V), at which a hole accumulation layer is formed in the silicon substrate beneath the transfer electrode. Generally, when the pinning voltage is taken as Vp, VL used is at −9.5 V that is lower by about 0.5 V than Vp. The pinning voltage is determined depending on the impurity concentrations of an N-type region and P-type region beneath the transfer electrode and the thickness of the gate insulating film (see, for example, Japanese Patent Laid-Open No. Hei 10-150184).
Next, a vertical charge transfer unit of CCD wherein charge transfer is carried out by four-phase drive is illustrated with reference to FIG. 15 that is a schematic sectional view of the charge transfer unit. As shown in FIG. 15, a vertical charge transfer unit of CCD for carrying out charge transfer by four-phase drive includes, for instance, a semiconductor substrate 111, on which transfer electrodes 152 (φ1), 152 (φ2), 152 (φ3) and 152 (φ4) are repeatedly formed successively through a gate insulating film 151. Moreover, an interlayer insulating film 153 is formed to cover these transfer electrodes 152 therewith. This interlayer insulating film 153 is formed of a silicon oxide film.
With a solid-state image pickup device having such a vertical charge transfer unit as set out above, a region to which a pulse voltage VL is applied is more intensely pinned, thereby reducing a dark current. However, there is a limit in further lowering the value of the pulse voltage VL (or increasing an absolute value). Even if the value of the pulse voltage VL can be made lower so as to keep the pinning state, consumption power increases and is thus disadvantageous for a solid-state image pickup device whose consumption power is ordinarily demanded to be low.